Digital transcription system utilizing small aperture acoustical sensors

ABSTRACT

A pen transcription system and method for using the same are disclosed. The pen transcription system includes a receiver having first and second acoustical sensors mounted on a planar base and separated from one another, an EM detector, and a controller. The first and second acoustical sensors detect an acoustical signal at a first wavelength emitted by a moveable signal source. The EM detector detects an EM signal that is synchronized with the acoustical signal. The controller measures the difference in time of detection between the EM signal and the acoustical signals detected by the first and second acoustical sensors. The acoustical sensors include a detector and a housing surrounding the detector, the housing having an aperture having a maximum dimension that is less than the first wavelength divided by 6.28.

BACKGROUND OF THE INVENTION

Acoustic-based distance measuring systems have been used to compute the position of a data entry object in a writing field for some time. For example, schemes that track and record the position of a pen on a white board are commercially available. As the user writes on the white board, the transcription system determines the location of the pen on the board and records the location for later use.

In such systems, a conventional marking pen of the type used with white boards is inserted into a housing that includes an acoustical transmitter and an infrared transmitter. As the user writes on the white board in the conventional manner, the transmitter sends a combination of acoustical and infrared pulses. Two receivers that are separated in space receive the signals generated by the housing. Each receiver measures the time difference between the time of arrival of the infrared pulse and the acoustical pulse to determine the distance of the housing from that receiver. These distance measurements are then combined to determine the position of the housing relative to the receivers.

Infrared is used for the light signals to avoid problems with background light in the area of use. The acoustical signals are typically in the ultrasound range so that the signals are beyond the human audible range. In addition, the higher frequencies provide better spatial resolution. Each acoustical receiver is typically constructed from a microphone such as ceramic piezo microphones, PVDF films, condenser microphones, electrets condenser microphones (ECMs), moving coil microphones, etc.

Unfortunately, the sensitivity of these devices as utilized in prior art systems is not completely omni-directional at ultrasound frequencies. The variation in angle with respect to each sensor over the range of positions of the pen on the surface can be relatively large. Hence, angular variation in the gain of the ultrasound receivers can lead to increased errors due to noise and variation in the trigger point on the ultrasound pulse as a function of angle. The latter type of error results in an error in the perceived delay time of the ultrasound signal, and hence, an error in the calculated distance from the sensor to the pen. In the extreme case, the microphone can have insufficient gain to detect the pen in some regions of a large writing surface. These gain problems can limit the size of the work surface that can be transcribed.

In addition to an omni-directional detection profile, the frequency response of the sensor is also important. Even at ultrasound frequencies, there are narrow band background ultrasound sources that can interfere with the reception of the ultrasound pulse from the pen. For example, some motion detectors utilize an ultrasound signal to detect an object moving within the field of view of the motion detector. These narrow band sources can have a signal strength that is sufficient to mask the ultrasound signal from the pen in the transcription system. The transcription system pen is typically battery powered, and hence, cannot compete with a motion detector that is powered from an AC power source and generates a signal having an amplitude that is sufficient to detect the change in frequency of the signal after the signal has been reflected from a moving object.

SUMMARY OF THE INVENTION

The present invention includes a pen transcription system and method for using the same. The pen transcription system includes a receiver having first and second acoustical sensors mounted on a planar base and separated from one another, an EM detector, and a controller. The first and second acoustical sensors detect an acoustical signal emitted by a moveable signal source at a first wavelength. The EM detector detects an EM signal that is synchronized with the acoustical signal. The controller measures the difference in time of detection between the EM signal and the acoustical signals detected by the first and second acoustical sensors. The first and second acoustical sensors each include a detector that generates an electrical signal in response to said acoustical signal at said first wavelength and a housing surrounding the detector, the housing having an aperture having a maximum dimension less than said first wavelength divided by 6.28.

In one aspect of the invention, the aperture has a plane that is defined by an axis and the acoustical sensor has a reception function that is symmetrical about the axis. The axis can be substantially parallel to or substantially perpendicular to the base surface depending on the particular embodiment being implemented. The controller determines a position for the moveable signal source.

In one aspect of the invention, the receiver is fixed relative to a work surface on which the moveable signal source moves. The receiver can be located on the edge of the work surface or within the work surface so that the receiver detects signals when the moveable signal source is on at locations on all sides of the receiver. The receiver can also include a detector that is utilized by the controller to determine upon which of the lines connecting the first and second acoustical sensors the moveable signal source is located. In one aspect of the invention, the detector is an EM detector that is directionally sensitive. In another aspect of the invention, the detector is a third acoustical sensor that is spaced apart from the first and second detectors and located off of the line connecting the first and second detectors. The third acoustical sensor can also be utilized to measure the height of the moveable signal source over the work surface.

In one aspect of the invention, the directionally sensitive EM detector includes two EM detectors that view different portions of the work surface. The signals from the two EM detectors are also combined to correct for background EM noise that is common to the two detectors.

In a still further aspect of the invention, one of the first and second acoustical sensors generates a signal that is proportional to the acoustic energy in the auditory acoustical band and the controller outputs a signal related to that signal. This signal can be utilized to record conversations in the vicinity of the receiver together with the position of the moveable signal source to provide a more complete record of a presentation made on the work surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art pen transcription system.

FIG. 2 is a cross-sectional view of an acoustic sensor 30 that can be utilized in a pen transcription system.

FIG. 3 illustrates an acoustic sensor that utilizes one aspect of the present invention.

FIG. 4 illustrates the reception pattern in an acoustic sensor in which D<λ/(2π).

FIG. 5 illustrates an acoustic sensor that monitors a pen as the pen moves to different positions on a work surface.

FIGS. 6A and 6B illustrate the manner in which a vertically mounted acoustic sensor provides an improved reception function.

FIG. 7 is a top view of a pen transcription system according to the present invention.

FIG. 8 is a top view of a pen transcription system according to another embodiment of the present invention.

FIG. 9 illustrates a receiver 90 that utilizes one aspect of the present invention.

FIG. 10 illustrates a receiver 100 that utilizes another aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can be more easily understood with reference to FIG. 1, which illustrates a prior art pen transcription system. Pen transcription system 20 operates in conjunction with an input device 22 that has a housing that accepts a standard marking pen for writing on a work surface 21 such as a white board. Pen transcription system 20 is attached to one edge of work surface 21. Input device 22 emits both an electromagnetic (EM) signal and an acoustical signal in response to the user engaging a switch on the input device. The acoustical signal is typically in the ultrasound frequency range of 30 KHz to 80 KHz. Pen transcription system 20 includes an EM receiver 26 and two acoustical receivers, 24 and 25. A controller 27 computes the distance from input device 22 to each of the acoustical receivers by measuring the difference in arrival time between the acoustical signals received at the receivers and the EM signal. The computed position is forwarded to client device 28 such as a computer for use in recording the material that was written on the work surface.

Refer now to FIG. 2, which is a cross-sectional view of an acoustic sensor 30 that can be utilized in a pen transcription system. Acoustic sensor 30 includes a housing 31 in which a microphone 32 is located. Housing 31 has an opening 33 having a diameter D. The reception pattern for acoustic sensor 30 depends on the relationship of D to the wavelength, λ, of the acoustical signal being detected. The reception pattern for the case in which D>>λ/(2π) is shown at 34. It should be noted that the pattern has a main lobe that is shown at 34 and side lobes 35. For an ultrasound signal at 40 kHz in air, λ/(2π) is approximately 1.4 mm. Prior art devices utilize acoustic sensors in which D is greater than this value, and hence, have reception (and transmission) patterns similar to that shown in FIG. 2.

This choice of aperture improves the overall sound detection by increasing the energy that enters the cavity; however, the penalty for this choice is a reception pattern with very high variability as a function of angle. Define the direction normal to opening 33 as the normal direction to the acoustic sensor. This direction is indicated by the arrow shown at 36 in FIG. 2. Prior art pen transcription systems utilize acoustic sensors in which the normal direction is parallel to the work surface. Hence, the sensitivity of the acoustic sensor can vary greatly as a function of the direction of the acoustical signal relative to the normal direction. For example, when the pen is located at position 37, the gain can be an order of magnitude lower than the gain realized when the pen is at location 38. This can lead to regions of the writing surface in which the pen's position cannot be reliably determined, particularly in the presence of background ultrasound sources.

Furthermore, if D>>λ/(2π), the dimensions of the cavity will be much greater than the wavelength of the ultrasound signal. As a result, the cavity can have a number of closely spaced resonant modes at frequencies near the frequencies of interest. These modes will depend on the specific shape of the cavity. The resonant modes alter the frequency response of the cavity, and hence, the frequency response of the sensor can vary significantly as a function of the frequency of the ultrasound signal. Such variations can interfere with schemes that depend on a broadband signal to avoid narrow band background noise. In addition, the specific resonant modes that are excited can vary with the angle of incidence of the signal from the pen relative to normal 36, which can result in further variations in the frequency response as a function of pen location on the work surface.

Refer now to FIG. 3, which illustrates an acoustic sensor 40 that utilizes one aspect of the present invention. In an acoustic sensor according to this aspect of the present invention D<λ/(2π). In this case, the sensitivity function is omni-directional with no side lobes as shown at 41. It should be noted that acoustic sensor 40 utilizes an acoustical sensor 42 that is substantially a pressure sensor, and hence, the reception function can be controlled by the shape and volume formed by the outer enclosure 43 and the size and thickness of the opening 33. As a result, a sensor having a size that is significantly greater than D can be utilized while maintaining the omni-directional reception pattern.

Another important aspect for an acoustic sensor according to the present invention is the bandwidth of the reception function of the acoustic sensor. The environments in which transcription systems operate often include narrow band acoustic sources in the ultrasound range. For example, some motion detectors utilize ultrasound sources. The signal strength from these background sources can be significantly greater than that from the pen in the transcription system, and hence, some strategy is needed to differentiate the signal from the pen from that of these background sources.

One scheme utilizes the bandwidth of the detected signal to make the distinction. Motion detection transmitters utilize very narrow band signals; hence, a detection scheme that requires a broadband signal can be utilized to distinguish between the background signal and the signal from the pen. In this aspect of the present invention, the pen generates a signal having a broadband and the pen transcription system includes a mechanism that excludes narrow band signals. For example, the ultrasound signal can be modulated to generate two sidebands. The detection system depends on detecting both sidebands, and hence, can distinguish between the ultrasound signal from the pen and that from a motion detector or other narrow band signal.

These schemes depend on the reception of the acoustic sensor being substantially flat over a band of frequencies that is as wide as the frequency band generated by the pen's transducer. The presence of peaks in the frequency response of the acoustic sensor created by mechanical resonances of enclosure 43 can interfere with this type of detection scheme. In one aspect of the present invention, an acoustic sensor utilizes a mechanical, electrical, or software filter to attenuate resonances introduced by enclosure 43 or acoustical sensor 42. For example, a mechanical quarter wavelength trap 44 or other baffle could be introduced into enclosure 43. Similarly, an analog filter 45 can be introduced into the output of acoustical sensor 42 or the transcription system can digitize the signals from the acoustic sensors and apply an appropriate digital filter to attenuate the undesirable resonances.

Undesirable mechanical resonances in enclosure 43 can also be attenuated or eliminated by altering the shape of enclosure 43 to eliminate closed paths having a path length that is an integral number of wavelengths of the resonant frequency in question. For example, the cavity can be designed such that the opposite walls of the cavity are not parallel to one another.

If D<<λ/(2π), the reception function of the acoustic sensor will be omni-directional and have equal sensitivity at all angles relative to the normal direction. Hence, reducing the size of the opening in the enclosure provides a more uniform gain as a function of the angle of incidence of the acoustical signal with respect to the normal of the acoustic sensor. However, the amount of signal energy that is coupled to sensor 42 is also reduced if D is reduced. Hence, there is a tradeoff between uniformity of gain as a function of angle of incidence and signal strength. To obtain higher overall gain while providing a gain function that has adequate reception over all of the angles of interest on the work surface, a D value that is characterized by a single lobe reception function that has some variation in gain as a function of angle can be necessary. Refer now to FIG. 4, which illustrates the reception pattern in an acoustic sensor in which the aperture in housing 53 has a maximum dimension D<λ/(2π), but the reception pattern has some degree of non-uniformity. The reception pattern for acoustic sensor 50 is shown at 51. The gain varies as a function of angle of the incident signal. For example, a signal incident at angle 55 is a higher gain than one incident at angle 56.

The degree to which this variation in reception gain as a function of angle alters the signal strength in an acoustic sensor depends on the orientation of the acoustic sensor relative to the work surface. Refer now to FIG. 5, which illustrates an acoustic sensor 61 that monitors a pen 62 as the pen moves to different positions on a work surface 60. In this example, the normal to acoustic sensor 61 is parallel to the surface of work surface 60. Hence, as pen 62 moves from position 63 to position 64 to position 65, the gain of acoustic sensor 61 decreases due to the angular variation in the gain of acoustic sensor 61.

One aspect of the present invention is based on the observation that the reception function is only a function of the angle between the normal to the acoustic sensor and the transmitter; hence, if the acoustic sensor is mounted such that the normal to the acoustic sensor is orthogonal to the work surface, the variations in reception gain resulting from a non-uniform constant reception function can be substantially reduced. Refer now to FIGS. 6A and 6B, which illustrate the manner in which a vertically mounted acoustic sensor provides an improved reception function. FIG. 6A is a top view of acoustic sensor 61 on work surface 60, and FIG. 6B is a side view of acoustic sensor 61 on work surface 60. Referring to FIG. 6B, a non-constant gain function results in the gain of acoustic sensor 61 being a function of angle 67; however, the gain will still be a constant, independent of angle 66 for any given value of angle 67. In the arrangement shown in FIGS. 6A and 6B, angle 67 is set by the height of the acoustical transmitter 68 in pen 62 and the distance between the acoustic sensor 61 and pen 62. Since the height of transmitter 68 is set by the physical dimensions of the pen, angle 67 is mainly a function of the distance from acoustic sensor 61 to the pen. Furthermore, the variation in angle 67 for various positions on the work surface is much less than the variations of the angle 67 over the work surface when the acoustic sensor is mounted with normal 69 parallel to the work surface.

Refer now to FIG. 7, which is a top view of a pen transcription system according to the present invention, which utilizes a vertically mounted acoustic sensor. Pen transcription system 70 tracks the position of a pen 72 on a work surface 71 in a manner analogous to that described above. Pen 72 emits a pair of pulses, an electromagnetic pulse in the infrared pulse together with an acoustical pulse. The pulse pairs can be emitted periodically or in response to some event such as the user pressing a button on the pen or the pen being in contact with work surface 71. The pulse pairs are detected by a receiver 73 that includes a first acoustic sensor 74 and a second acoustic sensor 75 that are separated by a predetermined distance and mounted such that the normals of the acoustic sensors are perpendicular to work surface 71. To maximize the size of work surface 71, receiver 73 is typically positioned along the side of work surface 71 at a location that is centered along one edge of work surface 71 such that the a line from acoustic sensor 74 to acoustic sensor 75 is parallel to that edge.

Receiver 73 also includes an infrared receiver 76. Receiver 73 determines the distance of pen 72 from each of the acoustic sensors by comparing the time of arrival of the infrared pulse with the acoustical pulse received by each acoustic sensor. The position of pen 72, or information from which the position can be computed, is output by receiver 73, typically to a computer that stores the information and recreates the scene drawn by pen 72 on work surface 71 or takes other actions based on the position motion measurements.

By adding one more infrared or acoustic sensor to the receiver, a pen transcription system having twice the working area can be constructed. Refer now to FIG. 8, which is a top view of a pen transcription system according to another embodiment of the present invention that utilizes this aspect of the present invention. Pen transcription system 80 is similar to pen transcription system 70 discussed above in that pen transcription system 80 includes a receiver 85 having first and second acoustic sensors shown at 87 and 88, respectively, that are mounted such that the normal to each acoustic sensor is perpendicular to work surface 81. Receiver 85 is mounted in the center of work surface 81, and hence, receiver 85 can receive signals from both sides of receiver 85 thereby effectively doubling the size of work surface 81 compared to work surface 71. Receiver 85 includes two infrared receivers 82 and 83. The direction from which each infrared receiver can receive signals is restricted by barrier 84 such that each infrared receiver can only receive signals from one half of the work surface. Hence, the side of the receiver on which the pen is located can be determined.

In the arrangement shown in FIG. 8, the distance computation is carried out using the two acoustic sensors and whichever of the infrared receivers received the signal. However, arrangements in which a single infrared detector is used in the distance computations and a separate directional infrared receiver is used to determine on which side of the receiver the pen is located could also be constructed. Refer now to FIG. 9, which illustrates a receiver 90 that utilizes this aspect of the present invention. Receiver 90 includes acoustic sensors 91 and 92, and an infrared receiver 93 that are used to compute the distance from a pen to each of the acoustic sensors when a pulse pair is received. Infrared receiver 93 can receive signals from both sides of receiver 93. A separate directionally specific infrared detector 94 is used to determine the location of the pen relative to receiver 90. In the embodiment shown in FIG. 9, detector 94 includes two infrared detectors 95 and 96 separated by a barrier that limits the field of view of each infrared detector to one half the work surface on which receiver 90 is placed.

In embodiments that include two infrared detectors that are positioned such that only one of the two detectors receives the infrared signal from the pen at any given time, the signal from the other receiver can be used to enhance the infrared signal detection by providing a measurement of the background infrared noise. Ambient infrared noise is introduced by lighting and other equipment that utilize infrared signaling such as remote controls and infrared communication links used to transfer data between various digital devices. These sources are usually located off of the work surface and at some distance away from the infrared detectors. Hence, each infrared detector receives a background signal that is the same as that received by the other infrared detector.

In one aspect of the invention, the signal from each infrared detector is combined with the signal from the other infrared detector to provide a signal for that detector that is corrected for the common background noise. For example, the controller in the embodiment shown in FIG. 8 could form a first difference signal by subtracting the signal from detector 82 from the signal from detector 83 and a second difference signal by subtracting the signal from detector 83 from the signal from detector 82. One of the difference signals will include a positive signal corresponding to the infrared pulse sent by pen, and the other will include a corresponding negative signal. By examining the difference signals, the controller determines the side of the receiver on which the pen is located.

The above-described embodiments utilize an arrangement in which the direction from which the infrared signal originates is utilized to determine on which side of the receiver the pen is located. However, embodiments that utilize an additional acoustical sensor to provide this information can also be constructed. Refer now to FIG. 10, which illustrates a receiver 100 that utilizes this aspect of the present invention. Receiver 100 includes acoustical sensors 91, 92, and 101 and an infrared receiver 93. The distance from a pen to each of the acoustical sensors is computed using the difference in the time of arrival of the infrared signal and the acoustical signal from the acoustical sensor for each of the acoustical sensors. Since acoustical sensor 101 is not co-linear with acoustical sensors 91 and 92, the distances from the acoustical sensors to the pen and the location of the pen relative to receiver 100 can be uniquely determined from the three distance measurements.

It should also be noted that having a third acoustical detector enables the transcription system to determine the location of the pen in three dimensions. That is, the height of the pen over the work surface can be determined. The height information can be used to determine if the pen is in contact with the work surface or positioned above the work surface. Accordingly, the controller can activate transcription only when the user is actually drawing on the work surface, i.e., the pen is in contact with the work surface or within some predetermined distance from the surface.

As noted above, the acoustical transmitter in the pen operates at an ultrasound frequency. The precise frequency depends on a number of factors. The frequency should be above the auditory range of human beings and domestic animals that are likely to be within range of the pen when the pen is operating. In addition, the frequency should be different from that of other ultrasound transducers in the area. Such transducers are often used in motion sensing devices that control lighting or burglar alarms. Finally, for any given power level in the pen, higher frequency transmitters tend to have shorter ranges. Hence, pen transcription systems that must measure position over larger work surfaces preferably utilize lower frequencies than pens that are required to operate over small area surfaces. In one aspect of the present invention, the pen includes an ultrasound transducer having a frequency between 30 KHz and 80 KHz, and more particularly between 40 KHz and 80 KHz.

As noted above, the ultrasound frequency is preferably chosen to avoid background ultrasound sources such as those used in motion detectors. In one aspect of the present invention, the controller measures the acoustical spectrum being received by the receivers in the absence of the pen to determine if there are competing ultrasound sources. In such embodiments, the frequency of the ultrasound transducer in the pen can be varied in some predetermined range of frequencies either continuously or discretely. If one or more background sources are detected, the pen is set to a frequency that does not overlap with the background sources in question. The frequency selection can be communicated to the user by the client device shown in FIG. 1 or by a separate frequency indicator on the receiver. For example, receiver 100 shown in FIG. 10 could include a display 110 that displays the frequency channel that the pen is to utilize. The user can then set the pen frequency accordingly.

In another aspect of the present invention, at least one of the acoustical sensors is sensitive to sound in the auditory frequency range, i.e., 50 to 20000 Hz. The controller utilizes this detector to provide a signal for recording the comments made by individuals within range of this acoustical sensor that can be included in the transcription record with the position of the pen as a function of time. A filter can be implemented, either in hardware or software, to remove sounds outside of this frequency range or a sub-range thereof that is used for the recording function.

The above-described embodiments of the present invention utilize light in the visible or infrared region of the optical spectrum. However, a source that emits electromagnetic radiation of other wavelengths could be utilized provided the emitted radiation pattern is directional. Accordingly, the term “light” will be defined to include any directional electromagnetic radiation unless a more specific range of wavelengths is indicated.

The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims. 

1. An apparatus comprising a receiver comprising: a base having a planar base surface; first and second acoustical sensors that detect an acoustical signal at a first wavelength emitted by a moveable signal source, said first and second acoustical sensors being mounted on said base and separated from one another; a first EM detector that detects an EM signal that is synchronized with said acoustical signal; and a controller that measures the difference in time of detection between said EM signal and said acoustical signals detected by said first and second acoustical sensors; wherein said first and second acoustical sensors each comprise: a detector that generates an electrical signal in response to an acoustical signal of said first wavelength; a housing surrounding said detector, said housing having an aperture, said aperture having a maximum dimension that is less than said first wavelength divided by 6.28.
 2. The apparatus of claim 1 wherein said aperture is defined by an axis, wherein said acoustical sensor has a reception function that is symmetrical about said axis.
 3. The apparatus of claim 2 wherein said axis is substantially perpendicular to said base surface.
 4. The apparatus of claim 2 wherein said axis is substantially parallel to said base surface.
 5. The apparatus of claim 1 wherein said controller determines a frequency in said predetermined frequency range to be utilized by said moveable signal source based on a measurement of acoustical energy as a function of frequency in said predetermined frequency range in the absence of said moveable signal source.
 6. The apparatus of claim 1 wherein said controller determines a position for said moveable signal source.
 7. The apparatus of claim 1 wherein said receiver is fixed relative to a work surface on which said moveable signal source moves.
 8. The apparatus of claim 7 wherein said receiver is located on an edge of said work surface.
 9. The apparatus of claim 7 wherein said receiver is located at a point that is internal to said work surface, said moveable source being located on either side of a line joining said first and second acoustical sensors, and wherein said receiver comprises a detector that is utilized by said controller to determine on which of said lines connecting said first and second acoustical sensors said moveable signal source is located.
 10. The apparatus of claim 9 wherein said detector is a second EM detector.
 11. The apparatus of claim 10 wherein said controller combines signals from said first and second EM detectors to generate a timing signal that is corrected for background EM noise that is common to both of said first and second EM detectors.
 12. The apparatus of claim 9 wherein said detector is a third acoustical sensor that is separated from said first and second acoustical sensors and located off of said line connecting said first and second acoustical sensors.
 13. The apparatus of claim 12 wherein said controller determines a height of the moveable signal source above said work surface.
 14. The apparatus of claim 1 wherein one of said first and second acoustical sensors generates a signal that is proportional to the acoustic energy in the auditory acoustical band and said controller outputs a signal related to that signal.
 15. A method for determining the position of a moveable signal source on a work surface, said method comprising: providing a receiver at a fixed location on said work surface, said receiver comprising: first and second acoustical sensors that detect an acoustical signal emitted by said moveable signal source at a first wavelength, said first and second acoustical sensors being mounted on said work surface and separated from one another; and a first EM detector that detects an EM signal that is synchronized with said acoustical signal; and measuring the difference in time of detection between said EM signal and said acoustical signals detected by said first and second acoustical sensors; wherein said first and second acoustical sensors each comprise: a detector that generates an electrical signal in response to an acoustical signal in a predetermined frequency range; a housing surrounding said detector, said housing having an aperture, said aperture having a maximum dimension that is less than said first wavelength divided by 6.28.
 16. The method of claim 15 wherein said aperture is defined by an axis, wherein said acoustical sensor has a reception function that is symmetrical about said axis.
 17. The method of claim 16 wherein said axis is substantially perpendicular to said base surface.
 18. The method of claim 16 wherein said axis is substantially parallel to said base surface.
 19. The method of claim 15 further comprising measuring the acoustical energy in said predetermined frequency range in the absence of said moveable signal source and communicating a frequency in said predetermined frequency range at which said moveable signal source is to transmit said acoustical signal based on said energy measurement.
 20. The method of claim 15 further comprising determining a position for said moveable signal source on said work surface.
 21. The method of claim 15 wherein said receiver is located on an edge of said work surface.
 22. The method of claim 15 wherein said receiver is located at a point that is internal to said work surface, said moveable source being located on either side of a line joining said first and second acoustical sensors, and wherein determining said position comprises determining on which side of a line connecting said first and second acoustical sensors said moveable signal source is located.
 23. The method of claim 22 wherein determining on which side of said line said moveable signal source is located comprises detecting said EM signal in a second EM detector that provides different signals when said moveable source is located on different sides of said line.
 24. The method of claim 23 further comprising combining signals from said first and second EM detectors to generate a corrected EM signal that is corrected for background EM noise that is common to both of said first and second EM detectors.
 25. The method of claim 22 wherein determining on which side of said line said moveable signal source is located comprises detecting said acoustical signal in a third acoustical sensor.
 26. The method of claim 25 further comprising determining the height of said moveable source above said work surface.
 27. The method of claim 15 further comprising recording a signal related to an output of one of said acoustical sensors in an auditory frequency band. 